Various forms of frequency modulation (FM) are commonly used in radio communications and more particularly in public safety radio services. When a received FM radio frequency signal is filtered using a filter that does not meet Carson's Rule for the bandwidth of FM signals, distortion of the phase information of the recovered signal will take place. In Carson's rule, the required filter bandwidth (BT) to recover 98% of the total power of the FM signal is given by the formula BT=2*ΔF+2*B, where ΔF is the peak frequency deviation and B is the bandwidth of the modulating signal. The distortion of the phase information will result in an inaccurate computation of the direct current (DC) component of the FM signal. Any algorithm, which requires an accurate DC component value for the recovered signal, will experience problems due to this distortion. As is further known in the art, the DC component is proportional to the frequency offset between the received signal and the transceiver operating frequency. Due to limitations of the transceiver's intermediate frequency (IF) filter design, the current implementation of this discriminator algorithm exhibits a phenomenon when the received signal is approximately +/−3.5 to 4 KHz offset in frequency from the transceiver operating frequency and of nominal deviation. This problem is even greater importance for signals that are low in signal strength. This received signal may be the profile of an interfering and undesired signal.
In these cases, the IF filtering of the I/Q samples for FM, violates Carson's Rule for the bandwidth required for FM signals. In this case the phase difference information in the discriminator buffer is distorted. When the signal strength of the received signal is weak and/or the frequency offset is very large, the discriminated data will become even further distorted. An example of this type of algorithm which requires an accurate DC component value, is one used in connection with automatic frequency control (AFC). The AFC algorithm uses the DC component value calculated from the distorted discriminator data. This distortion can lead to the AFC pulling the tunable transceiver off frequency when no frequency control was necessary. This has the result of negatively affecting the RF sensitivity of the transceiver.
In U.S. Pat. No. 5,963,851 (“Automatic Frequency Control System Using Multiple Threshold Levels and Method of Using Same,” Motorola, Inc.) which is herein incorporated by reference, a system and method for using certain thresholds for limiting the Automatic Frequency Control (AFC) is taught. The implementation of the Offset Detector of this invention is handled in a Digital Signal Processor (DSP). The Offset Detector works by extracting the DC component of the demodulated signal and passing it to the AFC processor. The discriminator algorithm computes the phase difference information accounting for phase “wrap around”. The algorithm relies on the axiom that a phase angle will “wrap around” a unit circle to −π if it increases beyond π, where “unwrap” is performed by adding 2π.
Using an IF filter that does not meet Carson's rule results in added distortion in the filtered I/Q samples since energy from the side lobes of the spectrum for the FM signal will be eliminated. The distortion of the I/Q samples of the FM signal can lead the discriminator to incorrectly compute a phase difference that “wraps around” when it should not have done so. A phase difference of a value that is slightly greater than 1 (for a positive frequency offset) will “wrap around” to a large, negative number. In a similar manner, a phase difference of a value that is slightly less than −1 (for a negative frequency offset) will “wrap around” to a large, positive number. The “wrap around” is still a necessity of the discriminator algorithm, as was shown earlier. However, “wrap around” that is caused by distortion of the FM signal (by an IF filter that does not meet Carson's rule) is not desirable.
In practice, the solution to this problem is not achieved by merely designing a better IF filter. There is an implied tradeoff since it is difficult to widen the bandwidth of the IF filters (to satisfy Carson's rule for all possible signals that we may receive) because the filters must also meet governmental standards for “adjacent channel rejection”.
Therefore, if one were to widen the bandwidth, it would also be necessary to sharpen the roll-off of the filter to meet adjacent channel rejection goals. This would require the use of more “taps” to obtain a filter that approaches the frequency response of an ideal “brick wall” filter. However, this would also increase processing power (MIPS) which results in greater current drain. In obtaining this goal, often the Carson's rule requirement is infringed upon. The decision is justified by acknowledging that “most” of the desired FM signals that are to be received by the FM transceiver, are of a smaller bandwidth (nominal deviation), and remain within the filter cutoff frequency. However, when an FM signal received by a transceiver is outside of the desired range of the filter, this type of distortion will occur. The distortion will affect other algorithms (like AFC) which will degrade the discriminated data.
Hence, the AFC processor will not retune the tunable transceiver for some DC component however the offset detector will calculate a DC component of some lesser value. The number of discriminated samples that are falsely represented as large negative/positive values will determine how much below/above the Offset Detector will calculate the reported DC component value. As the same signal gets weaker, more discriminated samples are represented as a large negative/positive number. In this situation, the DC component will be calculated as a lower/higher value than the true DC component (due to so many large negative/positive values being averaged in), and can even fall within the “hardware tuning threshold” that is described in U.S. Pat. No. 5,963,851. In this case, the AFC Processor will falsely retune the tunable transceiver towards this offset carrier. This is erroneous since the true DC component was actually outside of the “hardware tuning threshold”. The re-tuning of the tunable transceiver can actually adjust the tuned carrier frequency up to the “maximum correction threshold” as described in U.S. Pat. No. 5,963,851. This means that the transceiver will be tuned incorrectly in this situation, and will not be stopped until the “maximum correction threshold” is reached. This threshold is a protective mechanism so that AFC does not pull the tunable transceiver too far off frequency. However, it does allow for tuning changes of at least the “hardware tuning threshold” (of +/−2 kHz) in the implementation. Such tuning changes will degrade RF sensitivity of the tunable transceiver.
Thus, the need exists to provide an adaptive threshold to improve accuracy of the DC component calculation for use with an automatic frequency control in a two-way radio system to ensure proper operation of AFC tuning compensation during varying signal conditions. This technique enhances previous systems and methods of preventing signal degradation and/or distortion due to improper tuning using AFC in the radio transceiver.